{% for siteId in groupSiteIds %}
    {% if siteId == entry.siteId %}
        // currentSite = entry.siteId
    {% else %}
        // not the current site
    {% endif %}
{% endif %}

$event->rules[] = [
    'pattern' => 'form-builder/elements/<action:>/<id:d+>/<siteHandle:w+>',
    'route' => 'form-builder/element/<action>',
    'defaults' => [
        'id' => null,
        'siteHandle' => Craft::$app->getSites()->getPrimarySite()->handle,
        'action'    => 'index'
    ]
];

public function actionEditEntry(string $sectionHandle, int $entryId = null, int $draftId = null, int $versionId = null, string $siteHandle = null, Entry $entry = null): Response
if ($siteHandle !== null) {
    $variables['site'] = Craft::$app->getSites()->getSiteByHandle($siteHandle);

    if (!$variables['site']) {
        throw new NotFoundHttpException('Invalid site handle: '.$siteHandle);
    }
}